Containers of a chemical suspected to be chlorine were dropped by helicopters on rebel-held east Aleppo, causing breathing difficulties in some people, the Syrian Observatory for Human Rights and the local health authority say.
The Britain-based war monitor said its network of sources had reported seeing at least four barrel bombs dropped on the al-Qaterji and Dahrat Awad neighbourhoods, with the smell of chlorine filling the area on Tuesday.
Medical sources told the Observatory they thought the gas was chlorine.
The Aleppo Health Directorate for east Aleppo said there were a number of cases of breathing difficulties reported.
Earlier on Tuesday the Syrian army advised civilians in besieged east Aleppo to avoid going into the streets and to steer clear of militant positions, and urged rebels to stop firing into government-held western Aleppo.
On November 11, the executive body of the global chemical weapons watchdog OPCW condemned the alleged use of banned toxic agents by the Syrian government and by militant group Islamic State.
A 13-month international inquiry by the OPCW and United Nations concluded in a series of reports that Syrian government forces, including helicopter squadrons, were responsible for the use of chlorine barrel bombs against civilians.
Syrian authorities deny having used chemical weapons in the conflict. Islamic State has not commented.